In most wire cut electrical discharge processing devices, the wire electrode travels in a direction which is nearly vertical to the X-Y plane between a pair of wire guides while one of the wire electrode and work piece moves on the X-Y plane relative to the other. During processing, a voltage pulse is applied to a gap which is formed between the wire electrode and the work piece and the work piece is processed by the wire by generating a discharge in this gap. This type of wire cut electrical discharge processing device is used for precise processing applications.
The wire electrode, typically having a 0.1 to 0.3 mm diameter, is conveyed from a wire spool through a pair of wire guides disposed respectively above and below the work piece, via a device comprising a number of pulleys which apply tension to the wire. In addition, the wire electrode is conveyed to an appropriate recovery device, usually via a wire take-up device comprising a number of pulleys. The wire take-up device is controlled so that the traveling speed of the wire electrode is maintained at a set value. The tension applying device is controlled so that the tension applied to the wire electrode is a set value. The tension on the wire electrode is set, for example, so that it corresponds to the diameter and material of the wire electrode as well as the type of processing. If the tension is set to a large value, good straightness of the wire electrode between the pair of wire guides is achieved. However, excessive force may cause the wire to break, which is undesirable.